PRIOR ART
In the production of printed web materials, as for example through the use of a web fed offset printing press, it is known that the web leaving the last printing unit carries ink which is not yet dry and it is further known that the web after leaving the press is often subject to rupture or breakage. When such breakage occurs, it is the normal occurrence that the broken end of the web will wrap around one of the rolls of the last printing unit, usually the blanket roll, and cause damage. Even if no damage results from the web wrapping around the blanket cylinder, it is difficult and time consuming to remove the wrapped around web from the press. Therefore, it has been known in the prior art to provide apparatus which is designed to prevent a broken web from wrapping around cylinders that are contained in the printing unit itself. In most of the prior art the web carrying wet ink passes between a pair of spaced apart rolls, so that the ink is not smeared, which rolls are designed to move together when a sensor detects a break in the web. Typical of this form of apparatus are those described in U.S. Pat. No. 4,549,485 and in British Patent No. 1408176. In U.S. Pat. No. 4,549,485 there are provided a pair of opposed gripping rollers 3 and 15 which are spaced apart during normal operation of the press but which can be moved into contacting relationship with the web when the web is ruptured. The movement of the rolls toward each other is effected when a suitable sensor such as a web tension sensor, detects that the web has been broken. A somewhat similar arrangement is described in the British Patent which was noted above. In both the U.S. patent and in the British patent the rolls that grip the web are driven by some suitable outside driving apparatus.
In another type of apparatus, specifically that described in U.S. Pat. No. 4508033, there are provided a pair of opposed rolls 14 and 15 which are rubber covered and are in constant contact with the traveling web. In this case the rolls are driven at a peripheral speed which is slightly greater than the linear speed of the printed web so as to maintain tension on the web that is exiting from the printing press.
It has been determined that while apparatus of the type referred to above are somewhat effective they still are not totally effective because on the one hand the reaction to grip a broken end depends upon sensing that the web has already been broken so that the gripping rolls do not always advance the web coming from the press in a reliable fashion. In addition, all of the rolls are driven so that speed adjustments must constantly be maintained to insure that there is uniformity of rotational speed of the gripping elements and the linear speed of the web to maintain proper tension. In addition, where the rolls are in total contact with the traveling web there is the problem of smearing or otherwise disturbing the ink that has been printed on the traveling web.